Research in this Section consists of studies on the physical and chemical properties of proteins of biological interest and the roles of ligand binding and protein-protein interactions in enzyme catalysis and regulation. Interactions of divalent cations, substrates, and inhibitors with glutamine synthetase from Escherichia coli have been studied by calorimetry, ultracentrifugation, equilibrium dialysis, pH, spectral, thermal perturbation, and kinetic techniques. L-Methionine-SR-sulfoximine (a transition state analog) promotes both local and gross conformational differences between unadenylylated and adenylylated enzymes. This ligand also affects Mn2 ion and adenylyltransferase interactions with glutamine synthetase. Calorimetric and ultracentrifugal studies of the substrate-promoted conformational transition of aspartate transcarbamoylase from E. coli have been performed. The heats of assembly of this enzyme (C2R3) from catalytic chain trimers (C) and regulatory chain dimers (R) have been measured in the absence and presence of a bisubstrate analog.